Method for recovering and/or separating sulfur oxyfluorides from gas mixtures

ABSTRACT

A method for the recovery or separation of sulfur oxyfluorides from gas mixtures, in which the gas mixture passes through at least one adsorption stage or membrane stage containing or consisting of zeolites with a modulus &gt;10. The sulfur oxyfluorides fixed to the adsorbent may be made available for use after desorption. The sulfur oxyfluorides enriched through the membrane can be re-used directly.

CROSS REFERENCE TO RELATED APPLICATONS

[0001] This application is a continuation of international patentapplication no. PCT/EP01/01136, filed Feb. 2, 2001, designating theUnited States of America, the entire disclosure of which is incorporatedherein by reference. Priority is claimed based on Federal Republic ofGermany patent application no. DE 100 05 456.0, filed Feb. 8, 2000.

BACKGROUND OF THE INVENTION

[0002] The invention relates to a method for the recovery or separationof sulfur oxyfluorides from gas mixtures, in particular for the recoveryof sulfuryl fluoride (SO₂F₂) or thionyl fluoride (SOF₂) from air orexhaust air which is contaminated therewith.

[0003] Federal Republic of Germany patent application no. DE 197 08 669describes a method for the fumigation of spaces by introducing afumigation agent and continuously withdrawing the treatment atmosphere,wherein the fumigation agent can be separated e.g. in an adsorber andcan be re-used after desorption. Coal particles or aluminium oxideparticles are mentioned as adsorption agents, and inter alia sulfurylfluoride as fumigation agent. Sulfuryl fluoride can be and is used as apesticide. For example pests in fresh wood or wood used in buildings(woodworm, fungi), in storage spaces or in mills (granary weevils) ortextiles can be controlled using SO₂F₂. In this case, SO₂F₂ is used onits own or in combination with other known pesticides. Once fumigationhas ended, usually the SO₂F₂ is decomposed by heat or destroyed by lye.The secondary products produced thereby can frequently be disposed ofonly on special waste dumps.

[0004] There is a need to be able to work up gas mixtures of this typewith the aim of being able to re-use the sulfur oxyfluorides.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is to provide an improvedmethod for the separation of sulfur oxyfluorides from gas mixtures.

[0006] Another object of the invention is to provide a method forseparating sulfur oxyfluorides from gas mixtures in which virtually noenvironmentally damaging constituents of the gas mixtures pass into theatmosphere and the components which are separated out can be re-used.

[0007] A further object of the invention is to provide a method forseparating sulfur oxyfluorides from gas mixtures in which no secondaryproducts requiring disposal are produced.

[0008] These and other objects are achieved in accordance with thepresent invention by providing a method for the recovery or separationof sulfur oxyfluorides from a gas mixture comprising passing the gasmixture over at least one adsorption or membrane stage containing orconsisting of a zeolite having a modulus >10 and a pore diameter of 0.4to 0.7 nm, and thereafter recovering separated sulfur oxyfluorides.

[0009] According to the invention, gas mixtures containing one or moresulfur oxyfluorides are contacted with adsorbents and/or membranes forrecovery of the sulfur oxyfluorides and the sulfur oxyfluorides whichare separated out are sent for re-use, optionally after desorption,depending on requirements. The air which is low in or freed of sulfuroxyfluoride from the gas mixture can be let off into the atmospherewithout hesitation.

[0010] In particular, the term sulfur oxyfluorides within the scope ofthe invention is to be understood as including compounds of sulfur,oxygen and fluorine such as SO₂F₂, SOF₄, and SOF₂. Gas mixtures having asulfur oxyfluoride content from 5 ppmv or 20 mg/m³ or more may beseparated.

[0011] According to the invention, the gas mixture is passed through atleast one adsorption stage and/or membrane stage containing orconsisting of zeolites, which have an SiO₂/Al₂O₃ ratio, the so-calledmodulus, >10 preferably >100, a particle size >1 mm, preferably >2 mm,and a pore diameter of 0.4 to 0.7 nm, preferably 0.5 to 0.65 nm, theparticle size applying for the zeolite which is used as adsorbent.

[0012] Zeolites which have been subjected to dealumination in order toreplace the Al₂O₃ in the lattice by SiO₂, which thereby increases themodulus, are highly suitable. At the same time, this makes the polarforces weaker and thus the hydrophobicity of the zeolite is increased(W. Otten et al., Chem.-1 ng. Tech. 64 (1992) No. 10, pp. 915-925).

[0013] It is within the scope of the invention to use the zeolites bothas a loose bed in adsorber columns or as a membrane. Use of acombination of an adsorption stage and a membrane stage likewise iswithin the scope of the invention.

[0014] The adsorption is effected at a pressure of at most 15 atm. at21° C.

[0015] In one embodiment of the method according to the invention, pestcontrol with SO₂F₂ is performed in closed containers or spaces. TheSO₂F₂-containing atmosphere present as a gas mixture (0.05 to 10% byvolume SO₂F₂) in the containers or spaces is removed therefrom oncefumigation has ended and is contacted with the adsorbents and/or themembranes to recover the SO₂F₂. The air which is depleted in SO₂F₂ fromthe gas mixture can then be discharged into the atmosphere.

[0016] In another embodiment, the gas mixture is passed over two or moreadsorption stages and/or membrane stages.

[0017] The adsorbed sulfur oxyfluoride can be desorbed from the sorbentby increasing the temperature and/or reducing the pressure and be sentfor re-use.

[0018] The regeneration of the adsorbents (desorption) takes place inknown manner by supplying heat and/or lowering the pressure.

[0019] The desorption takes place in a temperature range from 20 to 300°C., preferably at a temperature >100° C. It has proved advantageous toallow the desorption to take place at reduced pressure. In oneembodiment, the adsorber column after heating is exposed to a technicalvacuum of up to 10⁻³ bar for desorption.

[0020] If the gas mixture additionally contains impurities, purificationby e.g. filtration or adsorption with other adsorbents may take placebefore the adsorption. It is likewise within the scope of the invention,first to free the gas mixture of water, e.g. by drying with suitabledrying agents, and then to pass it through the adsorbers or membrane.

[0021] The apparatus for carrying out the method according to theinvention comprises at least one adsorber with a bed of zeolites havinga modulus >10, preferably >100, a particle size >1 mm, preferably >2 mm,and a pore diameter of 0.4 to 0.7 nm, preferably 0.5 to 0.65 nm, and/orat least one membrane, consisting of or containing a zeolite having amodulus of 10, preferably 100 and a pore diameter of 0.4 to 0.7 nm,preferably 0.5 to 0.65 nm. A compressor is located before the adsorberor the membrane. The apparatus comprises at least one supply line forthe gas mixture, a line for removing the gas into the atmosphere andalso measuring, regulating and monitoring devices.

[0022] In a preferred embodiment, it has proved advantageous to use atleast two adsorber stages, which may each also comprise a plurality ofadsorber columns, since they permit continuous operation if connected inparallel. The alternate operation of adsorption and desorption isensured by suitable regulating and control devices.

[0023] In one embodiment of the invention, the gas mixture to beseparated is introduced into the adsorber stage by a pump or acompressor via a supply line.

[0024] The number and capacity of the adsorbers depends on whether a gaswith a high or low sulfur oxyfluoride content is to be treated. Thecapacity of the adsorbers can be increased, for example, by increasingthe adsorber volume of the individual adsorber columns or by increasingthe number of adsorber columns.

[0025] Before the depleted gas is discharged into the atmosphere, thecomposition of the gas is analyzed and monitored by a monitoring device,which may preferably comprise an infrared (IR) spectrometer.

BRIEF DESCRIPTION OF THE DRAWING

[0026] The invention will be described in further detail hereinafterwith reference to an illustrative preferred embodiment shown in theaccompanying drawing FIGURE, which is a schematic representation of aninstallation for carrying out the method of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0027] The accompanying drawing FIGURE shows a schematic representationof a plant with parallel adsorber stages each comprising two adsorbercolumns connected in series. This plant is very compact and is suitablefor construction, for example, on the bed of a heavy truck. Forsimplicity, the compressors before each adsorber column have beenomitted. Likewise, the heaters, vacuum pumps, measuring and regulatingdevices etc. are not shown.

[0028] The adsorber columns are filled with a loose bed of the zeolitesdescribed above.

[0029] The plant is laid out such that for continuous operation oneadsorber line adsorbs while the other adsorber line is undergoingregeneration.

[0030] The gas mixture at a pressure of at most 20 bar (abs.) isintroduced via line A into the adsorber 1. The gas mixture leaving theadsorber 1 is passed into the adsorber 2 via line B. Before the gasmixture leaving the adsorber 2, which contains no or only insignificantamounts of sulfur oxyfluoride, is discharged into the atmosphere, thecomposition of the gas mixture is analyzed and monitored in themonitoring device AE (preferably an IR-analysis apparatus).

[0031] Simultaneously the adsorbers 3 and 4, which are connected to eachother via line D, are regenerated, i.e. the adsorbed gas mixture isdesorbed.

[0032] The desorption takes place by pressure, and change intemperature, in that the adsorber columns are heated and/or the pressurein the adsorber columns is changed with the aid of vacuum pumps.

[0033] The desorbed gas mixture which is greatly enriched with sulfuroxyfluoride is collected in a storage container and is thus availablefor further use.

[0034] In another embodiment of the method the gas to be separatedpasses through a combination of a membrane separation stage and anadsorption separation stage. The pressure on the inlet side of themembrane is usually higher than the ambient pressure. A pump or acompressor is located before the membrane. The gas mixture to beseparated is fed at a pressure of up to 20 bar. The retentate leavingthe membrane stage, with an increased content of sulfur oxyfluoride,passes into a collecting container and can be re-used immediately e.g.after liquefaction with a compressor.

[0035] The permeate is passed into at least one adsorber for furtherseparation. optionally the permeate may be compressed before entry intothe adsorber.

[0036] It is likewise conceivable that the retentate of the firstmembrane stage is introduced into a second membrane stage. The permeateresulting from this second membrane stage can be recycled into the firstmembrane stage or be passed into the subsequent adsorber stages.

[0037] The membrane may be present in conventional form. The membranematerial contains or consists of a zeolite having a modulus >10,preferably >100, and a pore diameter of 0.4 to 0.7 nm, preferably 0.5 to0.65 nm.

[0038] The apparatus can also be constructed in mobile form, for exampleon the chassis of a motor vehicle, so that the method according to theinvention can be carried out in situ.

[0039] The following examples are intended to illustrate the invention,without limiting its scope.

[0040] For the following examples, the following zeolites were used asadsorbents: Pore diameter No. Trade Mark Type Modulus Particle size nm 1Zeocat ZSM5 400 2.5 mm 0.53 × 0.56; PZ-2/400 Na form 0.51 × 0.55 2Wessalith Pentasil >1,000 2 mm 0.6 DAZ F 20 MF 3 Sicolith 4A 2 1-2 mm0.4 400 granules 4 Grace 522 A 2.6 1.6-2.5 mm 0.5

EXAMPLE 1

[0041] An SO₂F₂/N₂ mixture containing 2.47% by volume SO₂F₂ was passedover an adsorber which contained a bed of 320 g adsorbent. ZeocatPZ-2/400 (manufacturer: Chemie Uetikon GmbH, Uetikon (Switzerland)) wasused as adsorbent.

[0042] The gas mixture was passed over the adsorbent for 54 minutes at athroughflow rate of 3.13 liters/minute until a maximum emissionconcentration of SO₂F₂ of 20 ppmv was attained. The adsorption capacitywas determined by differential weighing.

[0043] Result: 23.28 g gas was adsorbed, of which 18.78 g was SO₂F₂.

EXAMPLE 2

[0044] An SO₂F₂/N₂ mixture containing 2.47% by volume SO₂F₂ was passedover an adsorber which contained a bed of 360 g adsorbent. Wessalith DAZF20 (manufacturer: Degussa AG) was used as adsorbent.

[0045] The gas mixture was passed over the adsorbent for 44 minutes at athroughflow rate of 3.63 liters/minute until a maximum emissionconcentration of SO₂F₂ of 147 ppm was attained after 51 minutes. Theadsorber capacity was determined by differential weighing.

[0046] Result: 23.4 g gas was adsorbed, of which 17.73 g was SO₂F₂.

EXAMPLE 3: (COMPARISON EXAMPLE)

[0047] The test was performed analogously to Example 1 except thatSicolith 400 (manufacturer: Solvay) was used as the adsorbent.

[0048] Result: no measurable quantity of gas was adsorbed.

EXAMPLE 4:(COMPARISON EXAMPLE)

[0049] The test was performed analogously to Example 2 except that Grace522 (manufacturer: W. R. Grace, Nd. Worms) was used as the adsorbent.

[0050] Result: no measurable quantity of gas was adsorbed.

[0051] The results show that with zeolites which do not meet theselection criterion according to the invention in terms of modulus,particle size and pore diameter, no adsorption of e.g. SO₂F₂ ispossible.

[0052] The foregoing description and examples have been set forth merelyto illustrate the invention and are not intended to be limiting. Sincemodifications of the described embodiments incorporating the spirit andsubstance of the invention may occur to persons skilled in the art, theinvention should be construed broadly to include all variations fallingwithin the scope of the appended claims and equivalents thereof.

What is claimed is:
 1. A method for the recovery or separation of sulfuroxyfluorides from a gas mixture, said method comprising: passing the gasmixture over at least one adsorption or membrane stage containing orconsisting of a zeolite having a modulus >10 and a pore diameter of 0.4to 0.7 nm, and thereafter recovering separated sulfur oxyfluorides.
 2. Amethod according to claim 1, wherein the sulfur oxyfluorides areadsorbed on the zeolites and are recovered by desorption.
 3. A methodaccording to claim 1, wherein the sulfur oxyfluorides are accumulated ata membrane surface and are recovered directly from the surface of themembrane.
 4. A method according to claim 1, wherein a zeolite having amodulus >10, a particle size >1 mm and a pore diameter of 0.4 to 0.7 nmis used as adsorbent.
 5. A method according to claim 1, wherein azeolite having a modulus >100, a particle size >2 mm and a pore diameterof 0.5 to 0.65 nm is used as adsorbent.
 6. A method according to claim1, wherein a membrane comprising a zeolite having a modulus >100 and apore diameter of 0.5 to 0.65 nm is used for the separation.
 7. A methodaccording to claim 6, wherein the membrane consists of a zeolite havinga modulus >100 and a pore diameter of 0.5 to 0.65 nm is used for theseparation.
 8. A method according to claim 1, wherein adsorption ofsulfur oxyfluoride is effected at a pressure of at most 20 atm (at 21°C.).
 9. A method according to claim 1, wherein the gas mixture is asulfur oxyfluoride/air mixture having a sulfur oxyfluoride content of atleast 5 ppmv or 20 mg/m³.
 10. A method according to claim 1, wherein thegas mixture is contacted with at least two adsorption or membranestages.
 11. A method according to claim 1, wherein the sulfuroxyfluorides are desorbed from the zeolite by increasing the temperatureor reducing the pressure.
 12. A method according to claim 11, whereinthe sulfur oxyfluorides are desorbed from the zeolite by increasing thetemperature and reducing the pressure.
 13. A method according to claim11, wherein the desorption is effected at a temperature ranging from 20°C. to 300° C.
 14. A method according to claim 13, wherein the desorptionis effected at a temperature >100° C.
 15. A method according to claim11, wherein the desorption is effected at a reduced pressure rangingdown to 10⁻³ bar.
 16. A method according to claims 1, wherein the methodis carried out using a mobile apparatus.